Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for power generation or a fuel for domestic use. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the relatively large volume occupied by gaseous natural gas. Therefore, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, the expenditures associated with liquefaction are generally very high and liquefaction is not economical for formations containing small amounts of natural gas.
Formations that include small amounts of natural gas may be merely small natural gas fields or may include primarily oil, with the natural gas being a byproduct of oil production (“associated gas”). In the past, associated gas has typically been flared. However, current environmental concerns and regulations often discourage or prohibit this practice.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline, jet fuel, kerosene, and diesel have been decreasing and supplies are not expected to meet demand in the coming years. Fuels that are liquid under standard atmospheric conditions have the advantage that they can be transported more easily in a pipeline than natural gas, since they do not require the energy, equipment, and expense required for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted to form a mixture of CO and H2 (“synthesis gas” or “syngas”). This syngas generation usually occurs either by dry reforming, steam reforming, or partial oxidation, respective examples of which are set forth below for methane:CH4+CO2→2CO+2H2  (1) CH4+H2O→CO+3H2  (2) CH4+½O2→CO+2H2  (3) Reactions (1) and (2) are endothermic and reaction (3) is exothermic. Examples of syngas processes are disclosed in U.S. Pat. No. 6,402,989 to Gaffney and Gunardson, Harold, “Industrial Gases in Petrochemical Processing” 41-80 (1998), both incorporated herein by reference.
In the second transformation, known generically as hydrocarbon synthesis (e.g., the Fischer-Tropsch process), carbon monoxide reacts with hydrogen to form organic molecules containing carbon and hydrogen. Those molecules containing only carbon and hydrogen are known as hydrocarbons. Hydrocarbons having singly bonded carbons are known as paraffins and are particularly desirable as the basis of synthetic diesel fuel. An example of a Fischer-Tropsch process is disclosed in U.S. Pat. No. 6,333,294 to Chao et al., incorporated herein by reference.
Typically, a Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of weights. Thus, the product produced by conversion of natural gas, often called “syncrude,” commonly contains a range of hydrocarbons including light gases, gases, light naphtha, naphtha, kerosene, diesel, heavy diesel, heavy oils, waxes, and heavy waxes. These cuts are approximate and there is some degree of overlapping of components in each range.